Arc fault detector

ABSTRACT

An arc fault detector, as a stand alone device or in combination with a circuit interrupting device such as a ground fault interrupter (GFCI), protects from potentially dangerous arc fault conditions. The device utilizes a line side or load side series connected inductance having an air or magnetic core to generate the derivative di/dt signal of the arc current in the conductor. The derivative signal is fed to an arc fault detector where it is analyzed for the presence of arcing. The device can have two series connected inductors inductively coupled to each other such that the signal from one inductor is inductively coupled into the other inductor for coupling to the arc fault detector.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application and hereby claims priorityfrom U.S. patent application Ser. No. 11/738,783 filed on Apr. 23, 2007which is a continuation application which claims priority from U.S.application Ser. No. 11/295,277 filed on Dec. 5, 2005, which issued asU.S. Pat. No. 7,259,568 wherein that application is a continuation ofU.S. patent application Ser. No. 10/743,248 filed on Dec. 22, 2003,which issued as U.S. Pat. No. 6,972,572, the disclosures of theseapplications being incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to an apparatus and method for arc faultdetection and more particularly relates to an apparatus and method forboth a stand alone arc fault detector and an arc fault detector combinedwith a circuit interrupter device.

BACKGROUND OF THE INVENTION

Circuit breakers, fuses and ground fault circuit interrupters (GFCIs)are commonly used devices for protecting people and property fromdangerous electrical faults. Fatalities and loss of property caused byelectrical faults that go undetected by these protective devices stilloccur. One such type of electrical fault that typically goes undetectedare arc faults. Arcs are potentially dangerous due to the hightemperatures contained within them. Thus, they have a high potential ofcreating damage, mostly through the initiation of fires. An arc,however, will trip a GFCI only if it produces sufficient current leakageto ground. In addition, an arc will trip a breaker only if the currentflowing through the arc exceeds the trip parameters of thethermal/magnetic mechanism of the breaker. Therefore, an additional typeof protection device is needed to detect and interrupt arcs. An arcdetector whose output is used to trigger a circuit interruptingmechanism is referred to as an Arc Fault Circuit Interrupter (AFCI).

The causes of arcing are numerous, for example: aged or worn insulationand wiring; mechanical and electrical stress caused by overuse, overcurrents or lightning strikes; loose connections; and mechanical damageto insulation and wires. Two types of arcing can occur in residentialand commercial buildings: contact arcing and line arcing. Contact orseries arcing occurs between two contacts in series with a load.Therefore, the load controls the current flowing in the arc. Line orparallel arcing occurs between the conductors of a circuit or from aconductor to ground. In this case the arc is in parallel with any loadpresent and the source impedance provides the only limit to the currentflowing in the arc.

An example of contact arcing is illustrated in FIG. 1. The conductors114, 116 comprising the cable 110, are separated and surrounded by aninsulator 112. A portion of the conductor 114 is broken, creating aseries gap 118 in conductor 114. Under certain conditions, arcing willoccur across this gap, producing a large amount of localized heat. Theheat generated by the arcing might be sufficient to break down andcarbonize the insulation close to the arc 119. If the arc is allowed tocontinue, enough heat will be generated to start a fire.

A schematic diagram illustrating an example of line arcing is shown inFIG. 2. Cable 120 comprises electrical conductors 124, 126 covered byouter insulation 122 and separated by inner insulation 128.Deterioration or damage to the inner insulation at 121 may cause linefault arcing 123 to occur between the two conductors 124, 126. The innerinsulation could have been carbonized by an earlier lightning strike tothe wiring system, or it could have been cut by mechanical action suchas a metal chair leg cutting into an extension cord.

The potentially devastating results of arcing are widely known and anumber of methods of detecting arcs have been developed in the priorart. A large percentage of the prior art refers to detecting highfrequency signals generated on the AC line by arcs.

A wide range of prior art exists in the field of arc detection. Some ofthe prior art refer to specialized instances of arcing. For example,U.S. Pat. No. 4,376,243, issued to Renn, et al., teaches a device thatoperates with DC current. U.S. Pat. No. 4,658,322, issued to Rivera,teaches a device that detects arcing within an enclosed unit ofelectrical equipment. U.S. Pat. No. 4,878,144, issued to Nebon, teachesa device that detects the light produced by an arc between the contactsof a circuit breaker.

In addition, there are several patents that refer to detecting arcs onAC power lines that disclose various methods of detecting high frequencyarcing signals. For example, U.S. Pat. Nos. 5,185,684 and 5,206,596,both issued to Beihoff et al., employ a complex detection means thatseparately detects the electric field and the magnetic field producedaround a wire. U.S. Pat. No. 5,590,012, issued to Dollar, teachesmeasuring the high frequency current in a shunted path around aninductor placed in the line, which can be the magnetic trip mechanism ofa breaker. In a second detection circuit, proposed by Dollar, highfrequency voltage signal is extracted from the line via a high passfilter placed in parallel with any load.

Various methods can be found in the prior art to authenticate arcing andto differentiate arcing from other sources of noise. Much of the priorart involves complicated signal processing and analysis. U.S. Pat. No.5,280,404, issued to Ragsdale, teaches looking for series arcing byconverting the arcing signals to pulses and counting the pulses.

In addition, several patents detect arcing by taking the firstderivative or second derivative of the detected signal. For example,U.S. Pat. No. 5,224,006, issued to MacKenzie et al., and U.S. Pat. Nos.5,185,684 and 5,206,596, issued to Beihoff et al, disclose such adevice.

Blades uses several methods to detect arcs as disclosed in U.S. Pat.Nos. 5,223,795, 5,432,455 and 5,434,509. The Blades device is based onthe fact that detected high frequency noise must include gaps at eachzero crossing, i.e., half cycle, of the AC line. To differentiate arcingfrom other sources of noise, the Blades device measures the randomnessand/or wide bandwidth characteristics of the detected high frequencysignal. The device taught by U.S. Pat. No. 5,434,509 uses the fastrising edges of arc signals as a detection criterion and detects theshort high frequency bursts associated with intermittent arcs.

U.S. Pat. No. 5,561,505, issued to Zuercher et al., discloses a methodof detecting arcing by sensing cycle to cycle changes in the AC linecurrent. Differences in samples taken at the same point in the AC cycleare then processed to determine whether arcing is occurring.

A characteristic of arcing on a conductor is the occurrence of highfrequency signals which are different from the frequency (normally 60cycles) of the current for which the conductor is intended to carry.Electrical arcing produced by alternating voltage will extinguish eachtime the voltage across the arc drops below a value sufficient tosustain the arc, and will re-ignite each time the voltage across the arcexceeds the arc's minimum ignition voltage. The ignition voltage issubstantially proportional to the size of the physical gap that the arcmust traverse.

The extinction voltage tends to be lower than the ignition voltage. Whenthe arc gap is very large, the arc will be intermittent and unstable andwill tend to extinguish itself and re-ignite as conditions permit. Asthe gap becomes smaller, the arc becomes more persistent and eventuallyself-sustaining. When the gap becomes much smaller, the arc tends toself-extinguish by completing the current path. When the arc conductscurrent, it produces high frequency signals on the electricalconductors.

A number of systems that have been developed to detect arcing inbuildings do so by monitoring high frequency signals present on theconductors. One such method of detecting arcing is by an arc detectorthat detects the derivative of the signal on the conductor. Typically,such arc detectors employ, for example, current transformers to producesignals representative of the high frequency signals on the wiring beingmonitored. Current transformers both add to the manufacturing cost ofthe arc fault detector and, because of the size of the components,creates packaging difficulties. In addition, current transformers have alimited high frequency response and poor signal-to-noise ratio.

Accordingly, there is a need for an arc fault detector that providesimproved signal to noise ratio, improve high frequency response, that isrelatively economical to build and that is relatively small in size.

SUMMARY OF THE INVENTION

The arc fault detector of the present invention can operate either as astand alone Arc Fault Circuit Interrupter (AFCI) or in combination witha Ground Fault Circuit Interrupter (GFCI) to interrupt the flow ofcurrent to a load when an arc is detected. The combination device, knownas an arc fault circuit interrupter/ground fault circuit interrupter(AFCI/GFCI), can be realized by the addition of arc detection circuitryto a standard GFCI. An AFCI/GFCI device is a combination arc fault andground fault detector, which has the ability to interrupt a circuit andthereby prevent both dangerous arcing and ground fault conditions fromharming personnel or property. The term ‘circuit interrupting device’ isdefined to mean any electrical device used to interrupt current flow toa load and includes, but is not limited to devices such as Ground FaultCircuit Interrupters (GFCIs), Immersion Detection Circuit Interrupters(IDCIs) or Appliance Leakage Circuit Interrupters (ALCIs).

In the arc detector here disclosed, an inductor connected in series withat least the phase or neutral conductor monitors the current in the atleast one conductor to detect arcing such as line-to-line, line-toground, line-to-neutral or contact arcing. The signal from the inductoris the derivative (di/dt) of the current monitored and is fed to arcdetection circuitry which comprises a peak detector with decay, amicrocontroller with edge timing logic and a circuit interrupter. Theseries inductor can vary from a wire having a partial loop or bend tosix or more full loops and a core that is either of air or a magneticmaterial for generating the derivative signal, the di/dt signal, of thecurrent flowing through the conductor.

The present invention is capable of detecting arc faults on the lineand/or load sides of the device. Once processed, the peak amplitudes ofthe sensed di/dt signals are directed to a microcontroller whichanalyzes the signal for the presence of arcing characteristics. Uponidentifying a signal which indicates that arcing is present in aconductor, a trip signal is generated and fed to an interruptingmechanism which interrupts the flow of electricity to the load.

The circuit for the microcontroller can be placed on its own chip or onthe chip typically used in today's GFCI. When a single chip is used forarc detection and ground fault protection, it can be powered from thesame power supply that is used to provide power to the GFCI and, inaddition other components of the GFCI such as the mechanism forinterrupting the flow of current to the load when a fault occurs. Thiscombined approach results in reduced manufacturing costs as mechanicalparts of the GFCI device such as the trip relay and the mechanicalcontact closure mechanisms now serve dual purposes. In addition, addingthe arc detection circuitry to an existing GFCI is a logical enhancementof present day GFCIs.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, features, and advantages of the present invention willbecome more fully apparent from the following detailed description, theappended claim, and the accompanying drawings in which similar elementshave similar reference numerals.

FIG. 1 is a mechanical diagram illustrating an example of contact arcingin a current carrying conductor;

FIG. 2 is a mechanical diagram illustrating an example of line arcingbetween two current carrying conductors;

FIG. 3 is a block diagram of an arc detection system in accordance withthe principles of the invention;

FIG. 4 is a block diagram of another embodiment of an arc detectionsystem in accordance with the principles of the invention;

FIG. 5 is a circuit diagram of an arc detection circuit of theinvention;

FIG. 6 is a side view of the series inductor(s) and microcontrollerpositioned orthogonal to each other on a circuit board;

FIG. 7 is a top view of the series inductor(s) and microcontrollerpositioned orthogonal to each other on a circuit board; and

FIG. 8 is a circuit diagram of the second embodiment arc detectioncircuit in combination with a ground fault detector.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 4, there is disclosed an arc detection circuitconfigured to detect arcing such as neutral-to-ground, line-to-ground,line-to-neutral and/or contact arcing. Arc detection is based upon usinga series inductor to monitor the current for the occurrence of arcs inat least one of the conductors of an alternating current electricalcircuit such as shown in FIGS. 3 and 4. The circuit for monitoring arcsincludes a source of current (not shown) coupled to terminals 12 and 16.An inductor 20, normally of the same gauge wire as the conductor 18 iscoupled in series with conductor 18. The inductance of series connectedinductor 20 can be formed from a wire having as little inductance aswould occur from a bend of 15 degrees, to as much inductance as wouldoccur from six or more turns of 360 degrees each, and having an air ormagnetic core. The inductance of series connected inductor 20 isdependent, in part, on the magnitude of the potential required tooperate the arc detector circuit 24. A typical series inductor havingabout four complete turns each with a diameter of about 1.8 centimeterswas found to provide a voltage of about 5 volts in the presence ofarcing without adding any significant series impedance to the circuit.If desired, a clamp circuit 22 can be coupled in parallel with theinductor 20 to limit the maximum voltage that will appear across theinductor 20. A power supply 15 connected across the phase and neutralconductors 14, 18 upstream of the series inductor provides the lowvoltage power required to operate the various components of the circuit.Arc detector circuit 24 powered by power supply 15 is connected toreceive the di/dt potential from series inductor 20. More specifically,arc detector circuit 24 is coupled to receive the di/dt signal of thecurrent in the neutral conductor 18 from inductor 20 and analyze it todetermine if arcing is present. Upon determining that arcing isoccurring, a trip signal is generated by an appropriate control circuitwithin arc detector circuit 24 and applied via conductor 26 to circuitinterrupter 28. Accordingly, when arc detector circuit 24 detects theoccurrence of an arc based upon the signal produced by series connectedcurrent sensing inductor 20, a trip signal is applied to circuitinterrupter 28, which disconnects power to the load.

In addition, the trip signal can be fed to annunciation apparatus suchas an LED, a light emitting means such as a lamp, an audio means such asa horn or siren, a graphical or alphanumeric display, etc. to indicatethe occurrence of an arc.

Referring to FIG. 4, there is shown a circuit which is similar to thatof FIG. 3 with the addition of a second inductor connected in serieswith the phase conductor and a clamp connected in parallel with theinductor to limit the maximum voltage across the inductor. An inductor30, which may be of the same or different gauge wire as that of inductor20, is connected in series with phase conductor 14. Series connectedinductor 30 can be formed from a wire having as little inductance aswould occur from a bend of 15 degrees to as much inductance as wouldoccur from six or more turns of 360 degrees each, and having an air ormagnetic core. When the series inductor is comprised of a conductorhaving a bend of 15 degrees, or a portion of a full turn, the diameterof the bend or portion of the turn can be, more or less, aboutthree-quarters to one and a half Cm. The actual inductance that inductor30 (and inductor 20) has is determined, mainly, by the magnitude of theoutput potential needed to operate the arc detector circuit 24, while,at the same time, minimizing the impedance that is added to theconductor. Inductor 30 is positioned to be in close proximity toinductor 20 and inductively coupled, either through air or magnetically,to inductor 20. An inductive coupling of about 20 percent between theinductors 20, was found to provide good results. However, an inductivecoupling from 5 percent to as close to 100 percent as is possible can beused.

In the embodiment of FIG. 4, inductor 30 is similar to inductor 20 andinductor 20 is inductively coupled to inductor 30. The inductivecoupling between the two inductors is about 20 percent. The signal ininductor 30 which is inductively coupled into inductor 20 is fed to arcdetector circuit 24. Other than inductor 30 and clamp 32 located onconductor 14, the circuit of FIG. 4 is similar to the circuit of FIG. 3.However, in the embodiment here disclosed, about 20 percent of thesignal generated by inductor 30 is inductively coupled into inductor 20and then fed to arc detection circuit 24. In those instances where alarger signal is required, inductor 30 can have an inductance that islarger than that of inductor 20, the inductive coupling between theinductors can be increased, or a second arc detector circuit similar to24 can be coupled to phase conductor 14. A clamp circuit 32 can becoupled in parallel with inductor 30 to limit the maximum voltage acrossthe inductor 30. Coupling between the inductors 20, 30 can be achievedthrough air or by magnetic material such as a magnetic core or amagnetic circuit. With either of these techniques, inductive couplingbetween the two inductors can be enhanced or reduced and, if desired,the overall inductance can be increased or decreased. In addition, tominimize undesired coupling effects between the inductors, themicrocontroller and the circuit board electronics, the inductors, themicrocontroller and circuit board can be positioned orthogonally, asshown in FIGS. 6 (side view) and 7 (top view).

As noted above, the derivative (di/dt) signal of the current in theneutral conductor generated by the series inductor 20 is fed to arcdetector circuit 24 via conductor 34, and the derivative (di/dt) signalof the current in the phase conductor generated by the series inductor30 is inductively coupled into inductor 20 from which it is then fed toarc detector circuit 24 via conductor 34. Thus, arc detector circuit 24receives signals from inductor 20 and inductor 30 and, therefore,monitors the current in both the neutral and the phase conductors. Forpurposes of alternate channel sensitivity, the inductances of the twoinductors 20, 30 can be coupled to be either inductively additive orcanceling.

Also, the inductors 20, 30 can have inductances that are of equal ordifferent values. Thus, depending of the requirements of the circuit,the inductance of inductor 20 can be less than, equal to or greater thanthe inductance of inductor 30.

In another embodiment of the invention the series inductor 20 of FIG. 3is in the phase conductor 14 and ground is used as the return currentpath. In still another embodiment the series inductor is at least onewinding of a transformer.

Referring to FIG. 5, there is shown a circuit diagram of the embodimentshown in FIG. 4. Inductor 20 is connected in series with conductor 18and inductor 30 is connected in series with conductor 14. Power supply15 which receives power from the phase 14 and neutral 18 conductorssupplies the required potential to the arc detector circuit 24. Thepower supply shown has a capacitor 41 connected in series with a diode42, and this series network is connected across the phase 14 and neutral18 conductors upstream of the series connected inductors 20, 30. Thejunction of capacitor 41 and diode 42 is connected through diode 43 toan output terminal provided to supply the required potential to the arcdetector circuit 24. Connected between the output terminal of the powersupply and the neutral terminal is a capacitor 44 in parallel with aZener diode 45.

The arc detector circuit 24 includes a peak detector with decay 50 and amicrocontroller with edge timing logic 60. The Peak detector includes adiode 51 connected between an input terminal of the microcontroller withedge timing logic 60 and the neutral conductor 18 at a point downstreamof the inductors 20 and 30. A parallel circuit of a resistor 52 and acapacitor 54 is connected between the cathode terminal of diode 51 and aneutral terminal. The diode 51 of the peak detector provides a chargingpath for capacitor 54. The peak detector with decay provides signalsthat are representative of the derivative (di/dt) of the current inconductor 18 and conductor 14 and also serves to stretch any high speedpulses detected by the series connected inductors.

The microcontroller with edge timing logic 60 may be of the typedisclosed in U.S. Pat. No. 5,223,795, the entire disclosure of which isincorporated herein in its entirety by reference. Microcontroller withedge logic 60 produces a trip signal when a signal which represents anarc is received from the peak detector 50. More specifically,microcontroller 60 analyzes the signal received from the peak detectorto determine if arcing is present and, upon finding that arcing ispresent, generates a trip signal which is fed to circuit interrupter 28.Crystal 62 provides timing for the operation of the microprocessor.

The trip signal generated by the microcontroller is fed via conductor 65to the gate terminal of a triac 74 in circuit interrupter 28. Circuitinterrupter 28 includes a relay having two separate sets of contacts 71,72 and a coil 73. Contacts 71 are series connected with phase conductor14 and contacts 72 are series connected with the neutral conductor. Thecoil 73 of the relay is connected in series with the triac and thisseries network is connected between the phase conductor 14 and a neutralterminal. The gate terminal of the triac is connected through resistor75 to conductor 65 to receive the trip signals from the microcontroller60. A trip signal from the microcontroller primes the triac to conductwhich allows current to flow through the relay coil and open thecontacts 71, 72.

Referring to FIG. 8, there is shown an arc fault detector in accordancewith the principles of this invention in combination with a Ground FaultCircuit Interrupter (GFCI). The circuit 182, commonly referred to as ArcFault Circuit Interrupter/Ground Fault Circuit Interrupter (AFCI/GFCI)comprises two current transformers having magnetic cores 233, 234 andcoils 235, 236, respectively, coupled to integrated circuit (IC) 225which may comprise the LM1851 manufactured by National Semiconductor orthe RA9031 manufactured by Raytheon. AC power from the phase 14 andneutral 18 conductors is input to power supply circuit 15 whichgenerates power for the internal circuitry of the AFCI/GFCI device.

The series circuit of relay coil 218 and SCR 224 is connected betweenpower supply 36 and a neutral terminal, and the gate terminal of the SCRis coupled to the output terminal of SCR trigger circuit 236. The outputof pin 1 of IC 225 is the input to the SCR trigger circuit 236.

A diode 245 is coupled in parallel with coil 235 which is coupled topins 2 and 3 via resistor 247 and capacitors 239, 249. Pin 3 is alsocoupled to neutral via capacitor 251. Coil 236 is coupled to pins 4 and5 of IC 225 via capacitors 237, 238, and pin 4 is also coupled toneutral. Pin 6 of IC 225 is coupled to pin 8 via sensitivity resistor241 and pin 7 is coupled to neutral via time delay capacitor 243. Pin 8is also coupled to capacitor 222 and to resistor 221 and is connected topower supply 15.

Line side electrical conductors, phase conductor 14 and neutralconductor 18, pass through the transformers 233, 234 to the load sidephase and neutral conductors. Relay coil 218 is coupled to operatecontacts 231, 232, associated with the phase and neutral conductors,respectively, which function to open the circuit in the event a fault isdetected. The coil 218 of the relay is energized when the SCR 224 isturned on by a signal from the trigger circuit 236. In addition, thecircuit comprises a test circuit comprised of momentary push buttonswitch 228 connected in series with resistor 230. When switch 228 ispressed, a temporary simulated ground fault from load phase to lineneutral is created to test the operation of the device.

Inductors 20, 30 are coupled in series with conductors 14, 18 anddownstream of the input to the power supply 36. The two inductors areinductively coupled to each other and inductor 20 is connected to feed asignal representative of the derivative (di/dt) current in theconductors to the arc fault detector 24 as described above. Themicrocontroller of the arc fault detector can be a stand alone componentor it can be a part of the IC 225 of the ground fault circuitinterrupter. If the microcontroller is a stand alone component, the tripsignal generated by the microcontroller is fed to the SCR triggercircuit 236. If the microcontroller is a part of IC 225, the trip signalis the TRIG-GFCI signal from IC 225.

In the description of the embodiments of the invention here disclosed,either one or both of the series inductors 20, 30 can be primarywindings inductively coupled either through air or a magnetic core to acommon secondary winding or separate secondary windings connected tofeed received signals to the microcontroller. Thus, the series inductorsprovide the derivative (di/dt) of current flow and are the primary of atleast one current transformer. An inductor of the invention heredisclosed can be formed from a conductor having as little inductance aswould occur from a bend of 15 degrees to as much inductance as wouldoccur from six or more turns of 360 degrees each, and having an air ormagnetic core. The series inductance is connected in series with all orpart of the current flowing in the conductor, where the individual orcombined inductances of the windings are used to obtain directmeasurement of the derivative of current flowing in the conductor(s).

The two series inductors 20, 30 can have an inductance of between 0.1and 1,000,000 nanohenrys. Inductors having an inductance of between 0.1and 100 nanohenrys were made by putting a loop having a turn of lessthan 45 degrees in a 12 gauge wire. A loop having a turn ofapproximately 45 degrees in the conductor produced an inductance ofapproximately one nanohenry and, a loop of about four turns of 360degrees each having a diameter of about 1 Cm. formed an inductor havingan inductance of approximately 1,000,000 nanohenrys.

There is here disclosed a method and apparatus for detecting theoccurrence of arcing of a conductor. Improved resolution, signal tonoise ratio, derivative accuracy and high frequency response is obtainedfrom a direct measurement of the derivative of current flow. In theinvention, the inductor is connected in series with the line current tomeasure the derivative di/dt of the current flow. Low noise measurementis achieved by referencing the electronics to one side of the inductor,and having the electronics monitor the voltage on the other side of theinductor.

Line current surges generate magnetic flux through the series inductorwhich, in turn, can induce a magnetic flux in the surrounding electronicmaterial. Surface and sheet currents can also be induced in thesurrounding material, in response to the magnetic flux. By orienting theinductor to be orthogonal to the printed circuit board having theelectronics of the arc fault detector, the surface and sheet currents onthe circuit board itself, and in the electronics mounted on or coplanerwith the circuit board can be minimized.

If the derivative di/dt of the current becomes very high in magnitude,there may be an undesirable large drop of line voltage across theinductor. This can be avoided by clamping the maximum voltage dropacross the inductor with one of more diodes, Zener diodes, avalanchediodes, diacs, mov's, sidacs, transorbs, gas tubes, etc.

In those instances where sensitivity to the derivative of current flowon both the phase and neutral power lines is desired, a second inductorcan be located in close proximity and orientation to the first inductorsuch that flux coupling is achieved between the two inductors. Couplingcan also be achieved, enhanced or reduced by using magnetic materialeither in a core or a magnetic circuit, either of which will effectivelymodify the overall inductance. Thus, there is also disclosed a secondinductor flux coupled to the first inductor for alternate channelsensitivity where the two inductors can be either additive or cancelingand can be of different magnitudes.

In those instances where the electrical power distribution network isthree phase, a third series inductor can be coupled in series with thethird conductor to produce a voltage across itself related to thederivative of current flow in the third conductor and positioned toachieve flux coupling with the inductor in the first and/or secondseries inductor(s).

In devices that employ current measurements, space, which is usually ata premium in many device designs, can be saved by combining the seriesinductor that is sensitive to the derivative of current flow with theprimary of a current measuring transformer. In this manner, the sameinductor which provides direct measurement of the derivative of thecurrent flow can also function as the primary of the currenttransformer.

Where alternate channel sensitivity and current measurement are bothrequired, the two inductors can act as the primaries of a currenttransformer. In this embodiment, the flux induced in the transformercore from each of the two inductors should be either additive orsubtractive but, when subtractive, they should not fully cancel eachother.

Where alternate channel sensitivity and ground fault detection are bothrequired, the two inductors can together act as the primaries on aground fault differential transformer. In this embodiment, the couplingprovided by the transformer may or may not be the only coupling betweenthe two inductors and the flux induced in the transformer core for agiven current, from each of the inductors, must either fully of nearlyfully cancel.

Where alternate channel sensitivity and ground fault detection are bothrequired, the two inductors can together act as the primaries on aground fault transformer. In this embodiment, the coupling provided bythe transformer may or may not be the only coupling between the twoinductors. Therefore, the flux induced in the transformer core for agiven current, from each of the inductors, should be additive orsubtractive.

The arc detector here disclosed can be combined with other types ofcircuit interrupting devices such a GFCI, IDCI or ALCI to create amultipurpose device. In the case of a GFCI, the arc detection circuitrycan be placed onboard the same silicon chip typically used in today'sGFCI devices. In some instances, some of the pins of commonly used GFCIintegrated circuits can be converted for multifunction operation. TheAFCI can be powered from the same power supply that provides power tothe circuit interrupting device. This combined approach can result inreduced manufacturing costs as the mechanical parts of the circuitinterrupting device such as the trip relay and the mechanical contactclosure mechanisms will serve dual purposes. In addition, adding AFCIcircuitry to an existing circuit interrupting device is a logicalenhancement of such present day devices. In particular, it is logical toenhance a GFCI with AFCI circuitry since a GFCI can detect arcing incertain situations including any condition whereby an arc producesleakage current to ground.

The foregoing has outlined, rather broadly, the preferred feature of thepresent invention so that those skilled in the art may better understandthe detailed description of the invention that follows. Additionalfeatures of the invention will be described hereinafter that form thesubject of the claims of the invention. Those skilled in the art shouldappreciate that they can readily use the disclosed conception andspecific embodiment as a basis for designing or modifying otherstructures for carrying out the same purposes of the present inventionand that such other structures do not depart from the spirit and scopeof the invention is its broadest form.

1. Apparatus for detecting arcs on an electrical power distributionnetwork having at least two conductors comprising: a first seriesinductance circuit adapted to be coupled to a first of the at least twoconductors and disposed to produce a voltage across itself related tothe derivative of current flow in the first conductor; a second seriesinductance circuit adapted to be coupled to a second of the at least twoconductors and disposed to generate magnetic flux related to thederivative of current flow in the second conductor; a current measuringcircuit coupled to at least one of said first and second seriesinductance circuits and disposed to measure the current flowing in atleast one of said at least two conductors from a waveform of the voltageacross said at least one series inductance circuit.
 2. A method fordetecting an arc fault on a power distribution network comprising:producing a voltage across said series inductor, said voltage having awaveform which relates to a derivative of current flow in a conductor;determining a change in current over time (di/dt) via a series inductor;applying a trip signal in response to the presence of a sufficientvoltage produced across said series inductor.
 3. The method as in claim2, further comprising the step of determining when said waveform of saidvoltage across said series inductor is representative of an arc fault.4. The method as in claim 2, further comprising the step of generating adetection signal from said waveform when said waveform is representativeof arcing on the network.
 5. The method as in claim 2, furthercomprising the step of clamping excess voltage across said seriesinductor to limit excess voltage across said series inductor.
 6. Themethod as in claim 2, further comprising the step of determining thepresence of a ground fault.
 7. A Method for detecting arcs on anelectrical power distribution network having at least two conductorscomprising: producing a voltage across a first series inductancecircuit, said voltage being related to the derivative of current flow inthe first conductor; producing a voltage across a second seriesinductance circuit adapted to be coupled to a second of the at least twoconductors to generate magnetic flux related to the derivative ofcurrent flow in the second conductor; linking the magnetic flux of thesecond series inductance to the first series inductance; and determiningwhen a waveform indicative of arcing on the network is present andgenerating an arc detection signal when a waveform indicative of arcingis present.
 8. The method as in claim 7, further comprising the step ofdetermining the presence of a ground fault.
 9. Method for detecting arcson an electrical power distribution network having at least twoconductors comprising: producing a voltage across a first seriesinductance circuit, the first series inductance circuit being adapted tobe coupled to a first of at least two conductors, said voltage beingrelated to the derivative of current flow in the first conductor;producing a voltage across a second series inductance circuit adapted tobe coupled to a second of the at least two conductors to generatemagnetic flux related to the derivative of current flow in the secondconductor; determining when a waveform indicative of arcing on thenetwork is present, and generating an arc detection signal when saidwaveform indicative of arcing is present; measuring a current in atleast one of said first and second series inductance circuits to measurethe current flowing in at least one of said at least two conductors fromsaid waveform of the voltage across said at least one series inductancecircuit.
 10. The method as in claim 9, further comprising the step ofdetermining the presence of a ground fault.